Amatsu Ida is a first-year student at Kakyoin High School and a member of its Stride Club, where he serves as one of the team's runners. Born on June 30, his astrological sign is Cancer, and his blood type is O. Physically, he stands 174 centimeters tall and weighs 62 kilograms. He has magenta hair with a distinctive black strand behind his right ear, and silver eyes.

Ida is characterized by an arrogant and self-centered personality, acting as if he is above others and speaking with a great deal of confidence. His motto could be described as "unruly and self-absorbed," yet his most defining trait is that he possesses the exceptional talent to back up every boast. He is recognized as one of the strongest runners, and his natural abilities as an ace, or anchor, are a significant asset to his team. Having grown up in the United States, Ida is bilingual, but his unique way with language and phrasing is often difficult for the general public to understand. Within the story, he is frequently seen communicating with his teammate Toya Natsunagi, who seems to comprehend him without issue.

In terms of his role on the team, Ida is a formidable runner who exhibits remarkable acceleration and speed. While he originally filled the crucial anchor position, he later cedes this role to the more experienced Tomoe Yagami. Although his parkour skills are described as average, his sheer speed and powerful acceleration allow him to dominate during races, as demonstrated when he and his partner Takeru Fujiwara annihilated the course record for the Sapporo Dome Time Trial. His primary motivation is rooted in his own ability and dominance as a runner, and his development involves adapting his role within the team dynamic, shifting from being the central ace to supporting a different strategic formation.